Acid resistant austenitic stainless steel



2,865,741 ACID RESISTANT AUSTENITIC STAINLESS STEEL No Drawing. Continuation of 450,785, August 18, 1954. 6, 1957, Serial No. 694,728

4 Claims. (Cl. 75-125) application Serial No. This application November This application relates to acid resistant stainless steels and more particularly to forgeable stainless steels that have enhanced resistance to mineral acids such as sulphuric acid.

Stainless steels have heretofore been known which are resistant to corrosive attack by mineral acids. However, such steels have generally been used in cast form since they can only be processed to wrought form by taking severe losses in forging operations or by very special and expensive processing operations which seriously affected the size and shapes that could be so processed.

It is accordingly an object of this invention to provide austenitic stainless steels resistant to mineral acid corrosion which are readily forgeable.

It is a further object of this invention to produce forgeable stainless steels which have a satisfactory resistance to boiling 30% sulphuric acid.

It is another object of this invention to provide acid corrosion resistant stainless steels which are comparatively economical to produce.

According to the teachings of our invention, the foregoing objects can be achieved in steels of the following composition:

' Percent Carbon 0.1 to .10 Manganese .15 to 2.0 Silicon .75 max. Sulphur .04 max. Phosphorus .04 max. Nickel 23.0 to 27.0 Chromium 17.0 to 20.0 Molybdenum .80 to 1.5 Copper .80 to 1.5 Titanium .20 to 2.0

We have discovered that the relatively large amounts of silicon, i. e. over 1%, frequently used to obtain enhanced corrosion resistance are very deterimental to forging qualities, whereas with less than about .75% silicon, satisfactory forgeability is obtained. While less than 75% silicon is desirable, it is hard to obtain with present steelmaking techniques. The influence of silicon on forgeability is evident from the following Table I:

Table I Heat Mn S1 N1 Cr M0 011 Hot Forge N o. ability 06 1. 1 45 17. 56 16. 86 98 99 Satisfactory.

. 06 1. 0 85 18.00 17. 00 1. 00 1. 00 Fair.

.06 1. 0 1. 65 18. 00 17. 00 1. 00 1. 00 Upsatisfacory.

. 06 1. 2 2.00 22.00 17. 00 l. 00 1. 00 Do.

. 06 80 64 22. 75 16. 87 l. 02 1. 00 Satisfactory However, lowering the silicon content to develop satis- United States Patent factory forgeability lowers the corrosion resistance. We have discovered that the corrosion resistance can be restored and even improved by the addition of titanium in amounts over .20% and up to about 2%, particularly if the nickel content is increased slightly. If titanium is added in amounts over 2%, objectionable stringers are encountered. The effect of lowering silicon and addition of titanium on corrosion resistance to boiling 30% by weight sulphuric acid is shown in the following Table II. As can be seen reducing silicon increases the corrosion rate to more than .006 I. P. M. (inches penetration per mouth) which is regarded as unsatisfactory.

Table II Corroslon, Heat 0 Mn S1 'Ni- Cr M0 Cu T1 I. P. M No. Boiling 30% HzSOi Table III Corro- SlOIl, Heat 0 Mn 81 Ni Cr Mo Cu Ti I. P. M. No. Boiling 30% HzSOt However to adequately resist attack in boiling 50% by weight sulphuric acid, i. e. to reduce the I. P. M. to about .01% or less, higher molybdenum is required. This is shown in Table IV.

' Table IV Corro- 5101], Heat .0 Mn 51 Ni Cr Mo Cu T1 I. P. M. No. Boiling H2SO4 These higher molybdenum contents contribute to poor hot working characteristics. 7

Numerous experiments have shown that titanium is unique in respect to enhancing corrosion resistance. The following Table V shows that the elements such as vanadium, columbium, tantalum, tungsten, and zirconium which are sometimes considered equivalents to titanium do not react in the same way as titanium in this respect.

Table V Corrosion, Heat No. Mn Si Ni Cr Me On Others I. P. M Boiling I E 804 1. 57 21. 98 16. 74 1.02 1.00 37 Ti .0055 1. 29 .59 21. 82 17.11 1. 03 1.00 V. 0104 1. 28 53 22. 30 17. 32 1. 07 98 1.01 C 0084 1. 34 .54 21. 20 16. 90 1.03 .97 .76 W- .0107 1. 63 21. 20 16. 72 1. 03 97 1.20 Cb-i-Ta.-. 0096 1. 38 .58 22. 40 16. 80 1. 03 1. 01 .49 Zr 0199 In view of extensive tests, as exemplified by the forc going tables, we have found that considering the economies of manufacture, the following ranges are preferred:

This application is a continuation of my copending application Serial No. 450,785, filed August 18, 1954, now abandoned.

While we have shown and described several specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A readily forgeable austenitic stainless steel highly resistant to corrosion by mineral acids and characterized by an I. P. M. rate of less than .006 in boiling 30% sulphuric acid, said steel consisting of Percent Carbon .05 to .08 Manganese .75 to 1.50 Silicon .60 max. Phosphorus .04 max. Sulphur .04 max. Nickel 23.00 to 27.00 Chromium 17.00 to 20.00 Molybdenum .80 to 1.50 Copper .80 to 1.50 Titanium .30 to .50

with the balance iron and residual amounts of other elements.

2. A readily forgeable austenitic stainless steel highly resistant to corrosion by mineral acids and characterized by an I. P. M. rate of less than .006 in boiling 30% sulphuric acid, said steel consisting of with the balance iron and other elements in amounts which do not adversely affect the properties.

3. A readily forgeable austenitic stainless steel highly resistant to corrosion by mineral acids and characterized by an I. P. M. rate of less than .006 in boiling 30% sulphuric acid, said steel consisting of with the balance iron and residual amounts of other elements.

4. A readily forgeable austenitic stainless steel highly resistant to corrosion by mineral acids and characterized by an I. P. M. rate of less than .006 in boiling 30% sulphuric acid, said steel consisting of Percent Carbon .01 to .10 Manganese .15 to 2.00 Silicon .75 max. Sulphur .04 max. Phosphorus .04 max. Nickel 23.00 to 27.00 Chromium 17.00 to 20.00 Molybdenum .80 to 1.50 Copper .80 to 1.50 Titanium .20 to 2.00

Norris May 2, 1950 Post May 15, 1951 UNITED STATES PATENT OFFICE cmmcA-m 0F CORRECTION 1=vit1=3nt'No 2,865,741

. I December 23, 1958 Raymond Smith et a1 It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 1, line 38, -ior "Carbon .to 10'' read *- carbon- -1----' ,01 to .lo

column 2 line 12, for "mouth read month Signedand sealed this 19th day May 1959.

(SEAL) Attest:

M ROBERT c. WATSON Attesting- Officer Comnissioner of Patents 

1. A READILY FORGEABLE AUSTENITIC STAINLESS STEEL HIGHLY RESISTANT TO CORROSION BY MINERAL ACIDS AND CHARACTERIZED BY AN I.P.M. RATE OF LESS THAN .006 IN BOILING 30% SULPHURIC ACID, SAID STEEL CONSISTING OF 